A bent sub is often used in conjunction with a downhole motor and a drill bit in a drill string to build a predetermined angle of a borehole and thus allow for directional drilling. The bent sub provides a bend in the drill string above the drill bit. By orienting the bent sub in a selected bend direction, and choosing a bent sub with a selected bend, the drill bit will advance in the bend direction an amount determined largely by the selected bend. Various prior art bent subs are known, as for example U.S. Pat. No. 5,029,654 of Wilson et al, U.S. Pat. No. 5,125,463 of Livingstone et al, U.S. Pat. No. 5,343,966 of Wenzel et al and U.S. Pat. No. 4,667,751 of Geczy et al.
In the prior art, various techniques are used to establish the location of the drill bit in the borehole. For example, U.S. Pat. No. 4,667,751 describes a technique using a system of concentric stabilizers. In addition, it is known to use weld on kick pads on the drill string above the bent sub to kick the drill string over to one side of the borehole and allow build on the opposite side. However, these prior art techniques have disadvantages, such as difficulty in passing an enlarged area in a blow out preventor or, in the case of the weld on kick pads, potentially causing damage to the drill string during welding.
The inventors have provided an improved downhole sub for use with a bent sub in U.S. Pat. No. 5,638,910 issued on Jun. 17, 1997. The downhole sub includes upper and lower tubular sections whose centers are offset from one another in the bend direction of the bent sub. The downhole sub includes an intermediate tubular section disposed between the upper and lower tubular sections, with the bore of the intermediate section included within overlap between extensions of each of the bores of the upper and lower tubular sections. The axes of the upper and lower tubular section are parallel and offset between about 1/4 inch and 2 inches. The intermediate section grades gradually into the upper and lower tubular sections. The downhole sub is used by incorporation into a drill string above the bent sub and spaced from the bend sufficiently to allow the drill string to use the downhole sub as a lever to force the drill bit into the wall of the borehole.